1. Field of the Invention
The present invention relates to a microwave oven and, more particularly, to a control circuit for controlling a magnetron of a microwave oven with simplicity.
2. Description of the Prior Art
Recently, it has been known to provide a control system for a microwave oven, including a programmable digital logic control circuit which enables a user to program information into the digital logic control circuit. A large scale integrated circuit such as a semiconductor chip is necessarily required within the digital logic control circuit. An example of such a control system is shown in Fosnough et al U.S. Pat. No. 4,011,428 issued Mar. 8, 1977, entitled "MICROWAVE OVEN TIMED AND CONTROL CIRCUIT".
A further example of a control system for a magnetron related to the programmable digital logic control circuit was disclosed in the copending U.S. Pat. No. 4,149,056 assigned to the same assignee, patented Apr. 10, 1979, entitled "MICROWAVE OVEN WITH FOOD TEMPERATURE-SENSING MEANS", the disclosure of which is incorporated herein by reference. The corresponding Canadian patent application is Ser. No. 277,750, filed May 5, 1977.
The following is a circuit configuration of the control system described in the above U.S. Pat. No. 4,149,056.
FIG. 1 shows the circuit construction of a microwave oven including a control unit 80 employing a food temperature control of a food to be cooked. The circuit mainly comprises the control unit 80, a slipping connector 81, a temperature-sensing probe 82, a triac 83, and a microwave generator including a magnetron 84. The control unit 80 includes control circuitry of large scale integrated circuit 91 etc. Two relays 92, 93 are operated by the large scale integrated circuit 91 to control a current flow of a gate line 85 and a cook relay 94, respectively. The relay 92 is provided for switching the power supply and the relay 93 is provided for controlling the magnetron energy with ON-OFF switching through the actuation of a switch 106 connected to the triac 83.
A temperature signal detected by the temperature-sensing probe 82 is introduced into the control unit 80 via the slipping connector 81, said unit 80 functioning to control a predetermined cooking temperature of the food which is positioned in the microwave oven.
A gate signal through the gate line 85 developed from the control unit 80 triggers the triac 83 to control the microwave energy of the magnetron 84, said signal corresponding to the predetermined food temperature selected by a control panel. The triac 83 with a varistor 87 as a protective device controls a current flow to a primary winding of a high voltage transformer 86 connected to a winding of a heater transformer 88, said transformer 86 being connected to the magnetron 84 via capacitor 89 and a rectifier 90 for activating the magnetron 84, and said transformer 88 being connected to a filament of the magnetron 84. A power supply side includes a commertial power source 95, and a monitor switch 96 which is mechanically placed in its OFF condition when a latch door is closed and is mechanically placed in its ON condition when the latch door is opened. The power supply side further includes a primary interlock switch 97 and a secondary interlock switch 98, which are mechanically placed in the ON condition when the latch door is closed and are mechanically placed in the OFF condition when the latch door is opened through the use of the latch mechanism. That is, the interlock switches 97 and 98 function to allow the power supply to the remaining portions of the circuit only when the latch door is tightly closed.
A timer for cooking is set at a desired value through the use of a plurality of touch keys included within the control panel. When a coil of a cook relay 100 is energized by the relay 92 which is closed by a start switch 99 on the control panel, the energization is held and functions to close relay contacts 101 and 102. The coil of the cook relay 100 functions to close the relay contact 102 which provides a current path to an oven light 105 and to close the relay contact 101 which provides a current path to the high voltage transformer 86. A fan motor FM for driving a fan blade to draw cooling air through the base of the microwave oven cavity and the turntable motor TTM are further enabled.
A stop switch 103 on the control panel is activated by a latch of the latch door. When a open lever provided on the latch door is operated, the stop switch 103 opens, and the cook relay 100 is opened, which in turn, opens the cook relay contacts 101 and 102 to cut off current flow to the oven light 105 and the high voltage transformer 86. When a memory start switch 104 is activated to recall information in a memory bank, a memorized program in a memory-bank is conducted.
To energize intermittently the magnetron 84, the triac 83 shall control the energization of the magnetron 84 and the relay contact 101 select cooking time for the food by controlling the power supply to the primary winding of the heater transformer 88 and the high voltage transformer 86. The relay contact 101 is positioned in a pathway connected to the high voltage transformer 86 in addition to the fan motor FM, the turntable motor TTM, and the heater transformer 88. Therefore, the relay contact 101 must be resistant to a high voltage and large current. This results in the necessity of supplying the cook relay 100 with a large voltage and current for the activation of the relay contact 101. Therefore, the additional relay 92 shall be provided and energized in accordance with a small voltage and current developed from the large scale integrated circuit 91.
The following are exemplary values of the voltage and the current at the respective elements.
(i) The input of the relay 92 (namely, the output of a driver (not shown) of the large scale integrated circuit 91): 12 V, 30 mA.
(ii) The output of the relay 92 for energizing a switch connected to the cook relay 100: 120 V, 0.5 mA.
(iii) The enabling magnitude for switching the relay contact 101: 120 V, 11 A.
The relay 93, on the other hand, has the values of the voltage and current for energizing the switch 106 as follows.
(i) The input of the relay 93 (namely, the output of the driver for the large scale integrated circuit 91): 12 V, 30 mA.
(ii) The output of the relay 93: 120 V, 0.5 A.